3.5. Photometer sensitivity

The photometer sensitivity is driven by the foreground thermal noise emission,
mostly from the telescope and the electrical noise of the readout electronics.

The point-source sensitivities have been updated in HSpot 5.0 to the in-flight
measured sensitivities on deep fields maps.
However, the achieved sensitivity is a strong function of data
processing and scan strategy.

The given sensitivies in Table 3.2
refer to scan maps processed with a high-pass filtering, with a very short width
(typically a half-width of 16 in the blue/green bands and 25 in the red band)
to mitigate the effect of 1/f noise and gain/detector responsivity drifts in time,
at the expense of removing any extended emisison beyond the scale of this filter length.
Maps processed with MADmap, which preserve extended emission at any scale,
cannot reach this sensitivity yet.

Based on in-flight performance, we recommend that observers:

Use the medium (20"/sec) scan speed to get optimum point-source sensitivity.
The fastest (60"/sec) scan speed should be used if a wide area is to be mapped.

Use concatenated cross-scans for observations that cannot benefit from
high-pass filter reductions (e.g. fields with spatially complex, extended
and diffuse emission).
The cross-scans are also useful to obtain higher photometric accuracy for faint sources.

Table 3.2. PACS photometer sensitivity

central wavelength

70µm

100µm

160µm

scan mapping 1σ-1second (mJy)

30.6

36.0

68.5

mini-scan mapping 5σ-1hour (mJy)

4.7

5.5

10.5

To a first order the sensitivity in all mode scales with the inverse of the
square root of the on-source observation time.
This scaling is used for the sensitivities and S/N ratios reported by HSpot.

The on-array chopping technique is only used in point-source photometry mode :
the sensitivity reached is worse than in min-scan mapping mode as the 1/f noise
cannot be filtered out by chopping at 0.8Hz as efficiently as in the spatial modulation
of the scan mapping.
See chapter 4 for more information on the observing modes.